Leeds U.S. Pat. Nos. 2,777,824 and 3,055,297 disclose marking structures made of highly porous plastic material, the pores of which are of microscopic proportions and are filled with a marking fluid such as an ink. Structures made in accordance with the teaching of the Leeds patents have experienced a high degree of commercial success as hand stamps, stamp pads, and also ink rolls such as are used for applying ink to printing members in automatic printing equipment. Such structures are advantageous because of their long life, both in length of time and in numbers of operations or impressions, and because they operate well without the necessity of repetitive re-inking of the marking surface. Such structures apply ink in a uniform and reliable fashion. For example, hand stamps including plates made using such structures, which have various characters or designs molded at their surface, provide sharp and uniform impressions with high definition and uniformity. As stamp pads, such structures place a uniform layer of ink on a marking device, such as a rubber stamp, and are reliable over long periods of time. Similarly, as ink rolls, such structures provide uniform inking of printing members and exhibit fast recovery to facilitate repetitive use over long periods of time.
Known methods of making marking structures generally involve an initial step of making a mold having indentations or cavities corresponding to a relief pattern to appear on the finished marking structure. The mold is made of a material which can partially absorb marking fluid from the finished marking structure. Since it is necessary to use an excess amount of marking fluid in order to make the marking structure, the mold is often made from an absorbent material to remove the excess ink from the marking structure as it cures in the mold. In the alternative, a layer of absorbent material, such as sized felt, has been placed over the microporous marking structure in order to absorb excess ink, allowing a non-absorbent mold to be employed.
A premix containing a thermoplastic resin and a marking fluid, such as an ink, is then placed in the mold to fill the indentations and form a layer of premix therein. A mixer and dispenser are used to prepare a batch of premix. Often some of the premix is wasted because the entire batch is not needed, and substantial time and investment in equipment is required in order to prepare the premix for use.
The resulting premix-filled mold is then placed in a press, and subjected to heat and pressure sufficient to form the ink-impregnated microporous structure which is suitable for making a self-inking (self-replenishing) hand stamp. The open-celled, skinless nature of the microporous structure allows a small but generally constant flow of ink from the marking structure. Thus, as ink is removed from the surface of the relief pattern on the marking structure when an impression is made, ink from the interior of the marking structure flows to the surface in sufficient quantity to allow formation of further images having substantially the same brightness and clarity as the initial image.
Other methods are known for forming plastisol mixtures into ink-dispensing articles. In one such method, a flexible foam material is prepared in the absence of the ink. See, for example, Fujimura U.S. Pat. No. 4,306,498, issued Dec. 21, 1981.
The method of the invention utilizes a flexible dispensing pouch for marking fluids such as ink or premix. Pouches containing fluid materials are known, but in unrelated fields such as food dispensing. For example, small flexible packages of catsup and mustard are in wide use. The present invention provides a method for making microporous marking structures which eliminates some of the disadvantages of known methods, particularly as to preparation of the premix.